Adieu, Le Grand K: The kilogram to be redefined for the first time in 130 years

by Deborah Netburn, Los Angeles TimesMay 18, 20196 minutes

In a subterranean vault in a suburb of Paris lies a small, rarely seen metal cylinder known as Le Grand K.

For 130 years, this golf-ball-sized hunk of 90% platinum and 10% iridium has served as the international prototype kilogram. That means it was the single physical object by which all other kilograms across the planet were measured.

If microscopic contaminants in the air caused Le Grand K to grow a bit heavier, the kilogram itself grew a bit heavier. If a rigorous cleaning or small scratch caused it to become ever so slightly lighter, the kilogram itself became lighter as well. Indeed, it is estimated that over the course of its lifetime, Le Grand K has lost 50 micrograms of mass.

But the long reign of Le Grand K is about to come to an end.

Starting Monday, the kilogram will be redefined not by another object, but by a fundamental property of nature known as Planck's